1. Field of the Invention
This invention relates to the structure of a device for sensing moisture on a window using reflection of a light beam.
2. Prior Art
British Pat. No. 1,242,621 and German Pat. No. (Offenlegungsschrift 24 20 594) both disclose an optical sensor for detecting rain on an automobile's windshield. Both sensors operate on the principal of the reflection of light. That is, a pair of prisms are secured to an inner surface of the window and project a beam of light through one prism to the other prism using a path which extends in the interior of the window. One of the prisms detects light emerging from the window so that the intensity of light detected by the detecting prism is decreased in the presence of condensation on the outside of the window.
In operation, when a light beam enters a sheet of transparent material at such an angle that it is totally reflected at one of the inside surfaces, the beam will continue to travel to and fro inside the sheet until it can be refracted to the outside through another object contacting the transparent material of the sheet. The light source provides a beam of light which is projected into the window material at an angle so that the light is totally reflected at the other surface of the window and then back again to the entry surface and thus to and fro until it leaves the interior of the window through the second prism and impinges upon a photodetector. This will be the case for as long as the surface of the window is actually dry. However, as soon as the outside surface of the window has some water droplets thereon, the light beam traveling through the interior of the window is transmitted outwardly through the external surface and into the droplets. The beam is thus prevented from reaching the photodetector or at least the intensity of the reflected part of the beam is reduced. This causes the photodetector to operate an output signal which is used to indicate the presence of water or actuate a windshield wiper.
However, neither of these patents teach a simple, readily manufacturable, compact plastic package for the entire moisture sensor unit including light sources, lenses, light reflectors and light detectors.
U.S. Pat. No. 3,639,770 to Zizelmann teaches an optical sensor incorporating a light source and a light receiver and a light permeable mass of resin material. For the disclosed sensor device to function as a detector of moisture on a windshield, the device would have to be mounted on the outside of the windshield. This is undesirable because it exposes the sensor to the environment, hinders cleaning of the windshield, and prevent placement of the sensor in the wipe path.
U.S. Pat. No. 4,427,879 to Becher et al teaches an optical connector assembly having mating parts, each of which has a cavity for receiving a photoelectric device. The invention relates to optoelectronic connector assembly systems including electrical connector parts for housing optoelectronic transducers and to optical connector parts for housing terminal portions of a fiber optic light guide. A fiber optic light guide has an integral lens associated therewith. Formation of the lens is taught through the application of heat to the end of the optical fiber. Surface tension effects dictate the shape of the lens and do not teach a spherical shape internal to a device such as a moisture detector. Thus, even though there is a general teaching of forming a lens there is no teaching of an advantageous moisture detector structure for use in conjunction with the inside of a windshield.
Also, generally related to the transmission or detection of light are U.S. Pat. No. 2,977,842 issued to Duke, U.S. Pat. No. 3,540,025 issued to Levin et al, U.S. Pat. No. 3,917,411 issued to Schweizer and U.S. Pat. No. 4,221,962 issued to Black. Again, none of these patents teach or suggest a simple readily manufacturable compact plastic package for the entire moisture sensor unit.
It would be desirable to be able to form such a simple moisture detector which is readily manufacturable and can be easily mounted on the inside of a windshield. Further, it is desirable that such a detector be unaffected by the ambient light intensity. None of the above patents addresses this ambient light problem. For example, the above-cited British and German patents, in the configurations presented, allow ambient light a direct path into the light receiving element. Since ambient light will often be significantly greater in intensity than the self-contained light source, some sort of compensation would have to be employed so that the output of the moisture detector would be a true indication of moisture. These are some of the problems this invention overcomes.